I have a rowing machine (a WaterRower) with a water tank (picture). The tank has a few cracks on the top. I'm thinking of gluing a piece of plexiglass on it as a patch but was wondering if anyone had a better idea.

Add'l info: the tank is 23" in diameter and 9" tall. The plastic is ~3.5mm thick. The cracks are roughly 2" long. I'm not sure what caused them. The tank is normally sealed except for a bung hole in the top (visible in the 2nd photo) but it's currently in 2 pieces so I can work on it. I can't tell what the plastic is made of (no markings) but the tank has a little give when pressed. Here are 2 photos taken from the inside:

For a repair to be long-lasting it would help to know what caused the cracks. If it is basically old-age, that's different from stress cracks, which are different from someone having hit it with something. Do you know what kind of plastic it is (may have a little triangle somewhere with a recycling code)? How old is it? How flexible is it? Is it permanently sealed or does it open so you can add more water? Can you estimate the thickness of the plastic? Can you post a picture of the cracks in your own tank so we get an idea of size and stress? What are the tank dimensions?
– fixer1234Feb 8 '18 at 4:12